3rd party scales do not need to have an axis parameter anymore

Since matplotlib 3.1 PR 12831 scale objects should be reusable and therefore independent of any particular Axis. Therefore, the use of the axis parameter in the __init__ had been discouraged. However, having that parameter in the signature was still necessary for API backwards-compatibility. This is no longer the case.

register_scale now accepts scale classes with or without this parameter.

The axis parameter is pending-deprecated. It will be deprecated in matplotlib 3.13, and removed in matplotlib 3.15.

3rd-party scales are recommended to remove the axis parameter now if they can afford to restrict compatibility to matplotlib >= 3.11 already. Otherwise, they may keep the axis parameter and remove it in time for matplotlib 3.13.